1. Field of the Invention
The present invention relates to a frequency converter for performing low-frequency conversion of a carrier chrominance signal, in order to record a color video signal on a magnetic tape or the like.
2. Description of the Background Art
FIG. 1 is a block diagram showing a conventional low-frequency converter for a carrier chrominance signal, which is employed in a video tape recorder. Such a structure is disclosed in the MITSUBISHI DENKI TECHNICAL REPORT Vol. 60, No. 7, 1986, pp. 64-66, for example. Referring to FIG. 1, an input 1 receives a carrier chrominance signal having a color subcarrier frequency f.sub.sc and another input 2 receives a signal which is synchronous with a horizontal synchronizing signal having a horizontal synchronizing frequency f.sub.H, such as a signal obtained by triggering a one-shot multivibrator by the horizontal synchronizing signal.
The carrier chrominance signal is supplied to a switching circuit 3 generally called a burst gate. The burst gate 3 is opened only in a color burst signal period so that a color burst signal is extracted from the carrier chrominance signal to be supplied to a multiplier 5 of a phase-locked-loop (hereinafter referred to as PLL) circuit 4. The multiplier 5 multiplies the supplied color burst signal by an output signal from a voltage control oscillator (hereinafter referred to as VCO) 6 in order to perform phase detection. Only a low-frequency component of the phase detection output is supplied to a control input of the VCO 6 through a low-pass filter (hereinafter referred to as LPF) 7. Thus, the output signal of the VCO 6 is stabilized at the same frequency and phase by the color burst signal. This loop of the PLL circuit 4 is called an APC (automatic phase control) loop.
A signal, which is synchronous with the horizontal synchronizing signal, is received through the input terminal 2 and is fed to a multiplier 9 of another PLL circuit 8. The multiplier 9 multiplies this synchronous signal by a signal obtained through frequency division by a frequency divider 11 of an output signal from a VCO 10, in order to provide phase detection. Only a low-frequency component of the phase detection output is supplied to a control input of the VCO 10 through an LPF 12. When the carrier chrominance signal is in the NTSC (National Television System Committee) system and the video tape recorder is in the VHS recording system, the divisional ratio of the frequency divider 11 is selected at 1/320, for example. In this case, the output signal from the VCO 10 is stabilized in the same phase with the horizontal synchronizing signal at a frequency of 320 f.sub.H. This loop of the PLL circuit 8 is called an AFC (automatic frequency control) loop.
The output signal from the VCO 10 is frequency-divided by a frequency divider 13. The divisional ratio of the frequency divider 13 is selected at 1/8, for example, so that a signal having a frequency of 40 f.sub.H is obtained at its output. The frequency divider 13 also serves as a phase transition switching circuit in the VHS system, to rotate the phase of an output signal by 90.degree. for each horizontal synchronizing period and reverse the direction of phase rotation field by field. Such processing, generally called PS (phase shift) processing, is for preventing crosstalk from an adjacent track. PI (phase invert) processing is performed in the case of the beta system.
The output signals from the VCO 6 and the frequency divider 13 are inputted in a multiplier 14, which is called a sub-balanced modulator. The multiplier 14 multiplies the two signals having the frequencies of f.sub.SC and 40 f.sub.H, respectively, by each other, to output a signal including two frequency components of f.sub.SC .+-.40f.sub.H. Within this signal, only the frequency component of f.sub.SC +40f.sub.H is extracted by a bandpass filter (hereinafter referred to as BPF) 15, which is, in turn, inputted in a multiplier 16 called a main balanced modulator.
The multiplier 16 also receives the carrier chrominance signal which is supplied to the input 1. The multiplier 16 multiplies the two input signals having the frequencies of f.sub.SC and f.sub.SC +40f.sub.H by each other, to output a signal including two frequency components of f.sub.SC .+-.(f.sub.SC +40f.sub.H). Within this signal, only the frequency component of 40f.sub.H is extracted by an LPF 17. Thus, a low-frequency conversion chrominance signal in the VHS system is obtained. This low-frequency conversion chrominance signal is supplied to a recording head 18 of the video tape recorder in superposition with a luminance signal as is well-known.
When the carrier chrominance signal supplied to the input 1 includes jitter during dubbing, such jitter is also included in the color burst signal extracted from the burst gate 3 and the signal which is synchronous with the horizontal synchronizing signal supplied to the input 2. Therefore, the jitter of the carrier chrominance signal is cancelled, in the multiplier 16, by the jitter in the output signal from the BPF 15.
The conventional low-frequency converter for a carrier chrominance signal as hereinabove described has the following disadvantages:
(i) The structure is complicated with a large number of components, so that the size of the circuit is inevitably increased.
(ii) A large number of spurious components are contained in the output signal of the multiplier 14, which is the sub-balanced modulator. These spurious components are mainly frequency components of the sum of or the difference between the output signal from the VCO 6 and a spurious component in the signal having the frequency of 40 f.sub.H outputted as the result of multiplication of the signal which is synchronous with the horizontal synchronizing signal by the PLL circuit 8 and frequency division of the synchronous signal by the frequency divider 13. Such spurious components cannot be sufficiently removed by the BPF 15, and hence unwanted, noise causing components are generated in the output from the multiplier 16.
FIG. 2 is a frequency spectrum diagram of signals in the low-frequency converter shown in FIG. 1, wherein symbols B1 and B2 denote bands of BPF 15 and LPF 17, respectively. The output signal from the multiplier 14 includes spurious components in the vicinity of the frequency f.sub.SC +40f.sub.H, which are caused by a sixfold component of the signal having the frequency 40f.sub.H. That is EQU 40f.sub.H .times.6-3.58MHz=3.774MHz-3.58MHz=194KHz
and the spurious components EQU 4.21MHz.+-.194KHz and 4.21MHz.+-.2.times.194KHz etc.
result from intermodulation of the components 194 KHz and 4.21 MHz (f.sub.SC +40f.sub.H). This spurious components are converted into a low-frequency range and remain in the output signal of the LPF 17 as frequency component EQU 629KHz.+-.194KHz and 629KHz.+-.2.times.194KHz etc.
where 629 KHz is equal to 40f.sub.H. This frequency componets cause color noise during reproducing so that picture quality is deteriorated.